The tweet was sent by a Secret Service employee who believed they were tweeting under their personal Twitter handle. This sort of thing happens quite often with official social media accounts, and the Secret Service issued this statement to the Washington Post:

“We apologize for this mistake, and the user no longer has access to our official account. Policies and practices which would have prevented this were not followed and will be reinforced for all account users. We will ensure existing policies are strictly adhered to in order to prevent this mistake from being repeated, and we are conducting appropriate internal follow-up.”